To reduce the emissions of NOx (NO and NO2) and to use the natural gas fuel with high efficiency, engine manufacturers developed spark ignited reciprocating internal combustion engines (SI-RICE) that operated with very lean air to fuel ratios. In a lean mixture of air and fuel, there is more air present than necessary to fully combust the natural gas fuel. The presence of the excess air reduces the maximum temperature during the combustion period, which reduces the formation rate for nitrogen oxides (NOx). Nitrogen oxides in the atmosphere, under many conditions, cause the formation of ozone, which is harmful to plant and animal health. As a consequence, NOx emissions, or emission rates, are limited by regulation in many jurisdictions. As air-fuel mixtures become leaner (more excess air) NOx emissions are reduced. However, as the mixtures become leaner, the standard sparkplug is unable to reliably ignite the air-fuel mixtures.
To overcome this limitation a small pre-combustion chamber (PCC) is placed in or adjacent to the combustion zone in the engine. The air-fuel mixture in the PCC is controlled separately from the main combustion chamber, such that the mixture is made richer than the main combustion zone mixture. This enables a standard sparkplug to reliably ignite the mixture inside the PCC. As the mixture in the PCC expands, the hot, burning gases enter the main combustion chamber through several holes in the PCC. These burning gases are then able to more readily ignite the lean mixture in the main chamber than the standard sparkplug by itself. These gases expelled from the PCC have a relatively higher NOx concentration compared to the combustion products from the main chamber. The resulting NOx concentration in the exhaust gases comes from the sum of those produced in the PCC and the main combustion chamber. Since the PCC volume is only a few percent of the volume of the main chamber, the resulting NOx concentration is lower than can be achieved without a PCC.
Additional apparatus, including a gas admission valve and a separate fuel delivery apparatus such as the check valve and fuel regulator for the PCC as shown in FIG. 1, is required to ensure a richer air-fuel mixture in the PCC. The gas admission valve can fail to operate correctly and the PCC fuel delivery control can go out of adjustment, which requires engine adjustment or shutdown and gas admission valve replacement. To eliminate the need for a separate fuel system, PCCs using a standard sparkplug were designed to operate with the main chamber air-fuel mixture.
Some manufacturers have increased the compression ratio of engines with PCCs, as the very lean mixtures enable a more-efficient higher compression ratio to be used, compared with engines operated with a richer air-fuel mixture.
There are a number of natural gas engines currently in service having PCCs with the separate gas admission valves. In these engines, the additional apparatus required for the PCC design makes the engine more complex to optimize and more subject to malfunction than the open chamber engine with a standard spark plug. Hence, there is a need for a trouble-free implementation for engines with PCCs. For existing engines with PCCs and the associated apparatus, the innovations described in the following description can provide benefits to the users and owners of such engines.
Accordingly, systems and methods that enable emission reduction in natural gas engines remain highly desirable.
It will be noted that throughout the appended drawings, like features are identified by like reference numerals.